Today there is a quick global expansion of the use of high speed transportation vehicles e.g. trains. This causes a big challenge when it comes to wireless communications, and in particular when performing handover when travelling on such a train. The high speed of the trains, usually around 200-350 km/h, will put demanding time constraints on the wireless network when preparing and executing handover, due to the short period of time it takes for the high speed train to run through a cell. Normally, the whole handover procedure from the detection of a strong neighboring cell to the connection to the target cell may take about 500 ms, depending on different timer settings, such as Time-to-trigger, measurement period etc. If one assumes that the cell of a radio access node has a radius of 500 meters, a high speed train will pass through as cell in about 10 to 20 seconds leading to a high volume of signaling and thus take a large portion of user equipment, UE, resources and system node resources. If the wireless network fails to meet these high time constraints, it might cause a high dropping ratio or non-service from the network.
Another challenge when traveling at high speed, and especially when a larger number of users travel at high speed in the same transportation vehicle, e.g. a train, is that all connected users, i.e. connected UEs want to perform handover at the same time. This will cause a high volume of random access attempts and also overhead signaling. Depending on the number of users, this might lead to a bottleneck in the Random Access Channel, RACH. The users may thus experience frequent service interruptions or low-quality service during handover even if the handover is successful at the end.
In order to reduce frequent handovers of high speed, users attempts have been made to create a sector along the high speed railway having an area that is shaped along the moving direction of the train. There have also been attempts to use special parameter configurations to speed up the handover procedure. Even if such attempts have reduced the above described problems to some extent, the high speed of a train still requires a rather high handover rate of the UEs and there are still a large numbers of UEs that need to be handed over at approximately the same time.
Thus, there is a need to find new ways to improve cellular communications for passengers, i.e. UEs, travelling on high speed transportation vehicles.